Hi guys. last year I had a very odd neck injury that doctors couldn't even logically explain. To make long story short my neck is very weak and I was told that I should not do any headstands if I practice yoga or any hard falls and certain falls./rolls in martial arts, unless I can find a way to make my neck and shoulder muscles very strong. I am currently trying to decide on a dojo to join and while my schedule is making it difficult to commit for the next few months, I would like to take this time to strengthen myself so I can practice more safely and freely once I do.

I'm looking for advise on exercises that I may do to strengthen my neck.
My neck is flexible - at least that's a good thing! but I'm a girl so Strengthening is really the keyword here. Growing big muscles on my neck would however be horrible, so if you have any advise just keep in mind I'm not a guy who wouldn't mind growing big